The thermal cracking of paraffins to olefins, particularly lower paraffins such as C2-4 paraffins typically ethane and propane to corresponding olefins is an energy intensive process. It has been proposed to catalytically dehydrogenate lower paraffins in the presence of oxygen. Typically a support is impregnated with a liquid catalyst and dried for subsequent use. While these types of catalysts are useful they generally have a low productivity.
Dehydrogenation processes are widely used in modern refining and petrochemistry. Processes of synthesis of butadiene, isoprene, long-chain olefins are commercialized. However, the area of dehydrogenation of light alkanes remains to be underexplored and the processes are far from the commercial scale. The most advanced are the processes of oxidative dehydrogenation based on the use of transition metal oxide catalysts and a robust oxidant, such as oxygen or air. The oxidative conversion makes the process of dehydrogenation thermodynamically advantageous and decreases the reaction temperature as compared to non-oxidative processes (e.g. thermal cracking). The conversion of ethane, which is the second major component of natural gas, to ethylene requires development of new catalysts and processes.
Several catalytic systems are known in the art for the oxidative dehydrogenation of ethane. U.S. Pat. No. 4,450,313, issued May 22, 1984 to Eastman et al., assigned to Phillips Petroleum Company discloses a catalyst of the composition LiO—TiO2, which is characterized by a low ethane conversion not exceeding 10%, in spite of a rather high selectivity to ethylene (92%). The major drawback of this catalyst is the high temperature of the process of oxidative dehydrogenation, which is close to or higher than 650° C.
Rather promising results were obtained for nickel-containing catalysts disclosed in U.S. Pat. No. 6,891,075, 2005 issued May 10, 2005 to Liu assigned to Symyx technologies, Inc. At 325° C. the ethane conversion on the best catalyst in this series was about 20% with a selectivity of 85% (a Ni—Nb—Ta oxide catalyst).
The U.S. Pat. No. 6,624,116, issued Sep. 23, 2003 to Bharadwaj, et al. and U.S. Pat. No. 6,566,573 issued May 20, 2003 to Bharadwaj, et al. both assigned to Dow Global Technologies Inc. disclose Pt—Sn—Sb—Cu—Ag monolith systems that have been tested in an autothermal regime at T>750° C., the starting gas mixture contained hydrogen (H2:O2=2:1, GHSV=180 000 h−1). The catalyst composition is different from that of the present invention and the present invention does not contemplate the use of molecular hydrogen in the feed.
U.S. Pat. No. 4,524,236 issued Jun. 18, 1985 to McCain assigned to Union Carbide Corporation and U.S. Pat. No. 4,899,003, issued Feb. 6, 1990 to Manyik et al, assigned to Union Carbide Chemicals and Plastics Company Inc. disclose mixed metal oxide catalysts of V—Mo—Nb—Sb. At 375-400° C. the ethane conversion reached 70% with the selectivity close to 71-73%. However, these parameters were achieved only at very low gas hourly space velocities less than 900 H−1 (i.e. 720 h−1). Additionally the supported catalyst is prepared by impregnating the support and not by a dry co-comminution process of the present invention.
The most efficient catalysts were described in the patents by Lopez-Nieto J. M. and coworkers.
U.S. Pat. No. 7,319,179 issued Jan. 15, 2008 to Lopez-Nieto et al. assigned to Consejo Superior de Investigaciones Cientificas and Universidad Politecnica de Valencia, discloses Mo—V—Te—Nb—O oxide catalysts that provided an ethane conversion of 50-70% and selectivity to ethylene up to 95% (at 38% conversion) at 360-400° C. The catalysts have the empirical formula MoTehViNbjAkOx, where A is a fifth modifying element. The catalyst is a calcined mixed oxide (at least of Mo, Te, V and Nb), optionally supported on: (i) silica, alumina and/or titania, preferably silica at 20-70 wt % of the total supported catalyst or (ii) silicon carbide. The supported catalyst is prepared by conventional methods of precipitation from solutions, drying the precipitate then calcining. The patent does not suggest co-communition of the catalyst and a support.
Similar catalysts have been also described in open publications of Lopez-Nieto and co-authors. Selective oxidation of short-chain alkanes over hydrothermally prepared MoVTeNbO catalysts is discussed by F. Ivars, P. Botella, A. Dejoz, J. M. Lopez-Nieto, P. Concepcion, and M. I. Vazquez, in Topics in Catalysis (2006), 38 (1-3), 59-67.
MoVTe—Nb oxide catalysts have been prepared by a hydrothermal method and tested in the selective oxidation of propane to acrylic acid and in the oxidative dehydrogenation of ethane to ethylene. The influence of the concentration of oxalate anions in the hydrothermal gel has been studied for two series of catalysts, Nb-free and Nb-containing, respectively. Results show that the development of an active and selective active orthorhombic phase (Te2M20O57, M=Mo, V, Nb) requires an oxalate/Mo molar ratio of 0.4-0.6 in the synthesis gel in both types of samples. The presence of Nb favors a higher catalytic activity in both ethane and propane oxidation and a better production of acrylic acid. Preparation of molybdenum-vanadium-tellurium-niobium catalyst useful in oxidation involves drying a slurry of a ceramic inert carrier and metal ionic precursor species; then precalcination and calcination of the slurry. This art does not suggest co-comminuting the catalyst and the carrier.
The preparation of a Mo—Te—V—Nb composition is described in WO 2005058498 A1, published 30 Jun. 2005 (corresponding to U.S. published application 2007149390A1). Preparation of the catalyst involves preparing a slurry by combining an inert ceramic carrier with at least one solution comprising ionic species of Mo, V, Te, and Nb, drying the slurry to obtain a particulate product, precalcining the dried product at 150-350° C. in an oxygen containing atmosphere and calcining the dried product at 350-750° C. under inert atmosphere. The catalyst prepared exhibits the activity and selectivity in the oxidation reaction comparable to the non-supported catalyst. Again this teaches away from the co-comminution of the catalyst and the support.
Mixed metal oxide supported catalyst composition; catalyst manufacture and use in ethane oxidation are described in Patent WO 2005018804 A1, 3 Mar. 2005, assigned to BP Chemicals Limited, UK. A catalyst composition for the oxidation of ethane to ethylene and acetic acid comprises (i) a support and (ii) in combination with O, the elements Mo, V and Nb, optionally W and a component Z, which is ≧1 metals of Group 14. Thus, Mo60.5V32Nb7.5Ox on silica was modified with 0.33 g-atom ratio Sn for ethane oxidation with good ethylene/acetic acid selectivity and product ratio 1:1.
A process for preparation of ethylene from gaseous feed comprising ethane and oxygen involving contacting the feed with a mixed oxide catalyst containing vanadium, molybdenum, tantalum and tellurium in a reactor to form effluent of ethylene is disclosed in WO 2006130288 A1, 7 Dec. 2006, assigned to Celanese Int. Corp. The catalyst has a selectivity for ethylene of 50-80% thereby allowing oxidation of ethane to produce ethylene and acetic acid with high selectivity. The catalyst has the formula Mo1V0.3Ta0.1Te0.3Oz. The catalyst is optionally supported on a support selected from porous silicon dioxide, ignited silicon dioxide, kieselguhr, silica gel, porous and nonporous aluminum oxide, titanium dioxide, zirconium dioxide, thorium dioxide, lanthanum oxide, magnesium oxide, calcium oxide, barium oxide, tin oxide, cerium dioxide, zinc oxide, boron oxide, boron nitride, boron carbide, boron phosphate, zirconium phosphate, aluminum silicate, silicon nitride, silicon carbide, and glass, carbon, carbon-fiber, activated carbon, metal-oxide or metal networks and corresponding monoliths; or is encapsulated in a material (preferably silicon dioxide (SiO2), phosphorus pentoxide (P2O5), magnesium oxide (MgO), chromium trioxide (Cr2O3), titanium oxide (TiO2), zirconium oxide (ZrO2) or alumina (Al2O3). However, the methods of preparation of the supported compositions involve the procedures of wet chemistry (solutions are impregnated into the solid support and then the materials are dried and calcined).
The preparation of a supported catalyst usable for low temperature oxy-dehydrogenation of ethane to ethylene is disclosed in the U.S. Pat. No. 4,596,787 A, 24 Jun. 1986 assigned to UNION CARBIDE CORP. A supported catalyst for the low temperature gas phase oxydehydrogenation of ethane to ethylene is prepared by (a) preparing a precursor solution having soluble and insoluble portions of metal compounds; (b) separating the soluble portion; (c) impregnating a catalyst support with the soluble portion and (d) activating the impregnated support to obtain the catalyst. The calcined catalyst has the composition MoaVbNbcSbdXe. X is nothing or Li, Sc, Na, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, Y, Ta, Cr, Fe, Co, Ni, Ce, La, Zn, Cd, Hg, Al, Tl, Pb, As, Bi, Te, U, Mn and/or W; a is 0.5-0.9, b is 0.1-0.4, c is 0.001-0.2, d is 0.001-0.1, e is 0.001-0.1 when X is an element. The patent fails to teach or suggest a co-comminution of the catalyst and the support.
Another example of the low temperature oxy-dehydrogenation of ethane to ethylene using a calcined oxide catalyst containing molybdenum, vanadium, niobium and antimony is described in the U.S. Pat. No. 4,524,236 A, 18 Jun. 1985 and U.S. Pat. No. 4,250,346 A, 10 Feb. 1981, both assigned to UNION CARBIDE CORP. The calcined catalyst contains MoaVbNbcSbdXe in the form of oxides. The catalyst is prepared from a solution of soluble compounds and/or complexes and/or compounds of each of the metals. The dried catalyst is calcined by heating at 220-550° C. in air or oxygen. The catalyst precursor solutions may be supported on to a support, e.g. silica, aluminum oxide, silicon carbide, zirconia, titania or mixtures of these. The selectivity to ethylene may be greater than 65% for a 50% conversion of ethane.
The trend in the prior art is the formation of a catalyst by impregnating a porous support and then calcining. The resulting catalysts tend to have a fairly low times space yield.
The present invention seeks to provide a method of preparation of a supported active catalyst for oxidative dehydrogenation of ethane into ethylene that would exhibit a superior performance (activity, selectivity and productivity) as compared with the systems described in the prior art. The novel composite catalyst comprises the Mo—V—Nb—Te—O oxide composition containing the known in the art M1 phase and a solid support with the surface area in the range of 1-100 m2/g. The support may be silica, alumina, titania, zirconia, ceria, lanthana, magnesia, zinc oxide or a mixture thereof. The active composite catalyst is prepared by co-comminution of a mixture of the active Mo—V—Nb—Te—O oxide catalyst and a support so that the weight percent of the active phase is ranging from 10 to 99%. The resulting fine powder with the particle size ranging from 1 to 100 microns can be then pressed into pellets and crushed to collect the necessary fraction ranging from 0.1 to 1-2 mm or extrudates can be formed that can be further loaded in the plug-flow catalytic reactor.